1. Field of the Invention
The invention relates to a heat drilling process for the drilling of tunnels, deep wells, and exploration boreholes, wherein the profile of the tunnel or borehole is melted by means of a heat source and wherein the resulting molten rock is pressed out during the drilling process.
2. Brief Description of the Background of the Invention Including Prior Art
A full melting-drill method as taught in German Patent DE-PS-2,554,101 is concerned with the melting of rocks under pressure with hydrogen-oxygen mixtures as combustion fuel gases and to the pressing of a resultant fused rock mass into the sidewall rock with the aid of the so-called "Litho-Frac Mechanism" which works in the resultant rock melt like the well-known "Hydro-Frac Mechanism" in pressed water, applied in oil field revival.
Melting-drill methods of the Los Alamos Scientific Laboratory, USA, performed before the process according to German Patent DE-PS 2,554,101, were carried out according to this principle. However, in the case of the Los Alamos Scientific Laboratory process, the melt energy was drawn from an energy source disposed in the drilling device, namely from a core reactor or from an arc. The heat generated there was then transmitted via heat pipes to the melt head of the drilling device. The temperature present in the melt head melted the rock mass. In this form of indirect energy transmission onto the rock mass to be melted, the level of the appliable melt temperature is limited, on the one hand, by the energy sources themselves and, on the other hand, by the thermal loading capacity of the melt-head material. German Patent document DE-PS 2,554,101 further developed the process by one large step in that an oxyhydrogen gas flame, formed by a stoichiometric combustion of hydrogen and oxygen, serves as heat source which, with its heat of over 3,000.degree. C., acts directly on the rock mass and melts the rock mass. The drill head itself only feeds the fuel gases and the temperature of the drill head can thus be maintained at a lower temperature by several hundred degrees celsius by means of inner cooling. The thermal load of the drill head is thus markedly reduced and its service life is correspondingly increased. In principle, the melt process operates such that the rock mass is melted in the immediate proximity of the heat source by means of heat application. Due to the enormous temperature gradients relative to the neighboring rock mass, cracks are formed in the neighboring rock mass by the tremendous temperature stresses. This procedure is a so-called thermofraction procedure. In the melting-drill process, regardless of the type of heat source used, the resulting melt and fused rock mass is continuously pressed into the cracked rock mass in that the resulting cracks are enlarged under high pressure. This procedure is called Litho-Frac.
According to the melting-drill method of the German Patent DE 2,554,101, the pressure of the hydrogen-oxygen combustion fuel gases has to be increased with increasing borehole depth, since the shearing forces in the sidewall rock increase proportionally to the depth, based on the increasing overburden pressure in the sidewall rock. Therefore, higher pressures have to be created in the melted mass in the context of the "Litho-Frac"-mechanism, in order to be able to crack open the surrounding rock and in order to allow the excess melted rock mass to flow off into the cracks of the surrounding rock. This method cannot be continued where the limit of the technically acceptable pressure generation is reached in the combustible gas mixture during operation.
This melting-drill method of total displacement of drill core fused mass into the sidewall rock comes to a stop if, as depth increases, the equally increasing shearing forces of the sidewall rock rise to the value of the pressure in the combustible gases which can technically be achieved in practice. As the melt drill apparatus advances, the pressure in the fused mass can no longer overcome the shearing forces of the sidewall rock and so becomes greater than the pressure in the combustible gases.